Worthy of further investigation, the Stonehenge Hidden Landscape Project (which is geophysing the entire World Heritage Site in 5 years) is reported to have discovered two large pits, one near either end of the Cursus.

Apparently these pits are aligned so that as seen from the Heelstone they mark the positions of sunrise and sunset on summer solstice.

I've yet to find the precise positions of the pits reported anywhere, but the sunrise one must be some way from the eastern end because the Lockyer azimuth crosses the Cursus some 400m from the eastern terminal.

I've indicated this on the following image from Google Earth.

The green line is the N-S meridian through Stonehenge, the purple one is Lockyer's azimuth from the Heelstone (ie the direction of summer solstice sunrise).

Google Earth view of where a Cursus pit ought to be (roughly) to mark summer solstice sunrise as seen from the Heelstone along Lockyer's azimuth. Click the attachment below for a larger version.

I dug out the 2010 interim report from the Hidden Landscape Project and screengrabbed the geophys of the west end of the Cursus.

In the image below the red line is a quick and dirty summer solstice sunset line drawn from the Heelstone on azimuth 310° to get an idea of where any sunset pit would have to be.

There are three candidate anomalies in the geophys plot near this line - one in the middle of the Cursus and one each just inside the north and south banks.

West end of Cursus with geophys overlay and rough summer solstice sunset line, showing possible pit locations. Click the attachment below for a larger version.

Update 2: Based on a screengrab from the video from the team (NB: there is no audio), it looks like the west pit is the one to the north, next to the north ditch. That would make it below the horizon as viewed from the Heelstone, so it'd need to be a huge post (if indeed it did hold a wooden post). The pits are apparently 5m across, depth unknown.

Hmm - the azimuth of the (assumed) west pit from the Heelstone is over 311°, which is a degree north of where summer sunset is today.

That means the alignment must date from a time when the Earth's axial tilt was near its maximum of 24° 21' (but we've got to assume my overlay of the preliminary geophys results is exactly right, which it may not be - wish there were OS grid refs of the pits available right now).

We *might* be looking at pits that were dug around 8000BC - around the same time as the mesolithic postholes in the car park.
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simon